On Android, the notion of a home directory is not apparent to the user, and it is designed for only one user.

The userid and groupid mechanisms are used to enhance application security.  Each application has its own userid, and it is the job of the application to appropriately secure the data, which may be in the cloud or stored locally.

The model is intended to work even for applications which need to protect the data from the user.  Think of a medical records application, which tries to prevent inappropriate use of the data.

Android makes it pretty easy to store data pretty securely, avoiding many of the pitfalls of  traditional desktop systems.  Android also makes it pretty easy to authenticate and share data with other applications.  See details at:

* [Security and Permissions | Android Developers](http://developer.android.com/guide/topics/security/security.html)
* [How Android Security Stacks Up - Technology Review](http://www.technologyreview.com/communications/24944/?a=f)
* [Root access vs secure apps and the consensual security model - post by Nick Kralevich](http://groups.google.com/group/android-security-discuss/browse_thread/thread/835a5c3826cf7ffc?fwc=1#msg_3f88d56c7bfe4627)